In the later steps of IC chip fabrication, it is common to wire the device by creating an appropriate pattern of metallic conductors separated by a low dielectric insulator on the surface of the chip. The lower the dielectric constant of the insulator, the more densely the wiring can be packed without compromising electrical integrity. Because of its low dielectric constant and its convenience in fabrication processes, polyimide has become a material of choice.
A particularly attractive process for metallization of the preferred polyimide dielectric would include the deposition of the polyimide layer and patterning with a series of trenches followed by the deposition of a suitable highly conductive metal in the trenches to form the connections. In a particular and advantageous application of this process, the metal would be copper or a copper alloy and would be deposited by evaporation or sputtering. The excess metal overlying the polyimide between the trenches would then be removed by chemical-mechanical polishing (CMP) back to the polyimide. (See FIGS. 1 to 4.)
Unfortunately, the theoretically attractive process described above does not work in practice. There are two problems, both of which arise from CMP: (1) the copper is pulled out of the trenches by the CMP and (2) the surface of the polyimide is badly scored by the CMP. One way around the problems is to replace polyimide by a different dielectric such as silicon oxide or nitride. Copper is then less prone to being pulled out of the trench because it adheres better, and the oxide or nitride is less prone to mechanical damage by CMP. One drawback is that the dielectric constants of oxide and nitride are significantly higher than that of polyimide. This drawback could possibly be overcome by depositing a layer of polyimide and then coating the polyimide with oxide or nitride, but the deposition of oxide or nitride is done at temperatures that render polyimide an unattractive substrate because of outgassing and decomposition in the deposition tool. Moreover, there is very high stress between a polyimide and an oxide or nitride layer, so that the layers tend to crack and fail.
Thus there is a need for a process whereby a metallization can be dependably and reproducibly patterned in a polyimide layer.
There is a further need for a process that allows CMP of a metal that has been deposited on a polyimide substrate.